There are numerous desirable products made by extruding two or more substances together so that the substances become generally radially-layered. The process for making such products is generally referred to as coextrusion. In other words, a product is formed by extruding a first substance which circumferentially surrounds a second substance and, where desired, the second surrounds a third, and so on. Others have proposed methods and apparatus for making such products. For example, U.S. Pat. Nos. 4,402,898 and 4,208,175 disclose methods and apparatus for extruding radially-layered plastic tubing. U.S. Pat. Nos. 5,124,161; 5,120,554; 5,110,276; 4,900,572; 4,882,185; and, 4,648,821 disclose methods and apparatus for extruding center-filled food products.
Generally coextrusion involves a separate extruder for each substance to be combined into the multi-layered extrusion product. Each of these extruders flows the extrudable substance or "extrudate" into an apparatus such as a manifold. This apparatus presents flow paths to the various substances to direct them ultimately to a means which combines them and extrudes them as a multi-layered (usually radially-layered) product through an extrusion orifice. While not disclosed by some, others in the art cited above recognize a need for the apparatus or manifold to have multiple-extrusion orifices for higher production output.
However, problems arise in connection with producing radially-layered products, especially when multiple orifices are employed. For example, it can be difficult to achieve a product which is uniform from orifice to orifice. A primary reason for this is that the extrudable substance flowing from any given extruder must be divided in some manner to distribute the substance along a flow path to each extrusion orifice. With conventional apparatus, achieving uniform distribution to each orifice, requires separate adjustment to vary the flow path or flow rate leading to each given orifice. Such adjustments must be checked, and perhaps repeated whenever a new batch of extrudate is supplied from the extruder. Adjustments may also be required upon conditions changing, such as temperature or flow rate from the extruder. These adjustments are not desirable as they require the purchase, installation and maintenance of the flow varying apparatus. Also, valuable production time is consumed in making the adjustments and scrap may be generated until the adjustments are complete. For example, U.S. Pat. Nos. 5,120,554 and 5,110,276 teach the use of valves interposed in the flow path of the extrudates to vary and adjust volumetric flow.
Uniform distribution of extrudate is also impeded by the inherent necessity of introducing the flow path of inner extrudable substances within the flow path of the outer extrudable substances. To address this problem, others have proposed complex manifold structures employing spider plates and the like which attempt to uniformly divide outer substance by flowing into additional flow paths temporarily while the flow path of the inner substance can be interposed and brought into coaxial alignment, with the outer substance flow path. In these embodiments, the outer substance is generally then recombined before it proceeds to the extrusion orifice. The apparatus proposed for this dividing and recombining flow is complex and further impedes uniform distribution. The complexity also increases equipment capital and maintenance costs. Examples of these structures are disclosed in the above-cited patents.
While presenting an improvement over the above discussed approaches and while enjoying significant commercial success, U.S. Pat. No. 4,900,572 to Repholz nonetheless discloses, among other things, the need for adjustment of flow and distribution of extrusion. For example, in Repholz, the extrudate flow and uniformity are adjusted by separately turning individual eccentric sleeves in each of the extrusion orifices.
Generally uniform distribution of extrudable substance is also frustrated by non-symmetrical and non-equal flow paths presented by conventional apparatus from the extruders to each of the multiple extrusion orifices.